Instruction Manual

DE
DANIELS
ELECTRONICS
2-2 VT-3A130 Amplifier Instruction Manual
The part of the output RF energy is coupled by a directional detector circuit TL1 which provides a
means of linearization of the audio and leveling of the output power over the frequency range 118
MHz to 138 MHz.
The AC coupled audio signal is applied to the inverting input of U4a in order to provide
predistortions to the input modulation audio signal, which is supplied from the audio amplifier U2
to the non-inverting input of U4a.
On the contrary the DC coupled part of the detected audio is passed through the 50Hz low pass
filter U6a, U6b, and than compared with a certain DC level which is set by R19. The difference of
the signals is amplified by U4b and applied to pin 2 of the Power Amplifier Module in order to
keep the output power relatively constant over the frequency range of 118MHz to 138MHz.
2.3.1 VT-3/150 Lowpass Filter
The VT-3/150 Output Lowpass Filter is a 50 ohm, 6 pole, reciprocal filter with a 3 dB cutoff
frequency of approximately 150 MHz. The low pass filter assembly attenuates the desired signal's
harmonics as well as any other out-of-band emissions so that a 'clean' RF signal is output to the
antenna connector.
2.4 Power Supply Circuitry.
The VT-3A130 Amplifier has shutdown circuitry based on Q2 that allows decrease power
consumption, less than 0.1 mA from 9.5Vdc, in standby mode. The voltage level on the amplifier
enable line of less than +2.0 Vdc turns on the transistor Q2. Q2 supplies the base circuitry of Q1,
Q3, Q4, and operational amplifiers U2, U4, and U6.
The U3 LM2588 used as a step-up (boost) regulator. This is a switching regulator that produces an
output voltage greater than the input supply voltage. The following is the LM1084ADJ, a low
dropout voltage (LDO) positive adjustable regulators with a maximum dropout of 1.5V at 5A of
load current. The LM1084 used as a postregulator that reduces the ripple current and also prevents
the output voltage level from overshooting the maximum of 16Vdc.